1. Field of the Invention
This invention pertains generally to the determination of precision angle-of-approach in multi-beam radar systems and more specifically to the pre-detection sensing and quantization of differential phase of an radio frequency wave impinging on two or more antennas to provide precision angle-of-arrival.
2. Description of the Related Art
Multi-beam systems incorporating differential phase and/or time-of-arrival for angle-of-arrival (AOA) estimation rely primarily on post-detection methods for the encoding of delta time-of-arrival (D.sub.TOA) or differential phase. These simultaneous multi-beams are parallel processed by using either detected-amplitude and D.sub.TOA comparisons or detected differential phase. Current- art multi-beam D.sub.TOA systems typically require large antenna spacing, due to inherent limitations in post-detection TOA measurement. Differential phase systems (interferometers) measure differential electrical phase through radio frequency (RF) mixing and integration, which can be severely contaminated by multipath or other phase distortions.
In conventional techniques the D.sub.TOA of a received signal from an antenna-pair spaced a given distance (d) can be calculated by ##EQU1## where d is the antenna separation in meters, c the velocity of light (299.70532.times.10.sup.6 m/s) and .theta. is angle-of-arrival.
Since current-art D.sub.TOA systems incorporate post-detection time-of-arrival sensing, quantization and resolution of time-of-arrival measurement is typically on-the-order of 50 ns. Further, use of post-detection affects the measured arrival time due to non-linearity and response limitations of the amplitude detector. If 50 ns is the quantization limit for a typical D.sub.TOA system, a desired AOA accuracy of .theta.&lt;1.0 spatial degrees requires the receiving antenna-pair to be separated by approximately 865 meters. Due to the inability of current systems to measure time-of-arrival with high precision and subsequently the need for very large antenna spacing, current D.sub.TOA systems are best suited for multi-satellite, multi-aircraft, or radio astronomy instrumentation.
Additionally, differential phase systems are also quite limited. Multipath is a major contributor to this limitation. Incident RF waves that approach a differential phase system are joined a small time later by reflected waves. This causes distortions in both phase and amplitude at the antenna system. Depending on the geometry involved, these distortions can be observed only nanoseconds after the leading pulse edge. Current-art interferometers can not properly obtain AOA information under these circumstances.